Exposure apparatus are commonly used to transfer images from a reticle onto a semiconductor wafer during semiconductor processing. A typical exposure apparatus includes an illumination source, a reticle stage assembly that positions a reticle, an optical assembly, a wafer stage assembly that positions a semiconductor wafer, a measurement system, and a control system. The measurement system monitors the position of the reticle and the wafer, and the control system controls each stage assembly to adjust the positions of the reticle and the wafer. Because the features of the images to be transferred from the reticle onto the wafer can be extremely small, precise positioning of the wafer and the reticle is critical to the manufacturing of high quality wafers.
In certain designs, measurement systems include an autofocus system that is used to map the height of the wafer surface along an optical axis prior to exposing the wafer. Subsequently, with information regarding the position along the optical axis, the wafer stage assembly can be controlled to properly position the wafer along the optical axis.
In one type of autofocus system, a fringe pattern is projected onto a substrate, and shifts of the fringe pattern can be used to estimate substrate displacements along the optical axis so that focus errors can be corrected during exposure. The fringe pattern can be generated by irradiating a diffraction grating and imaging the diffraction grating onto the substrate. After exiting the diffraction grating, a 0th order diffraction component can be blocked, while components associated with +1 and −1 diffraction orders are transmitted to the substrate. Typically, the fringe pattern projected onto the substrate is imaged onto a detector such as a focal plane array detector, and lateral shifts of the fringe pattern on the detector are used to generate a focus adjustment. Such autofocus systems are described in Smith et al., U.S. Patent Application Publication 2012/008150, Smith et al., U.S. Patent Application Publication 2011/0071784, and Sogard et al., PCT International Publication No. 2012/177663, all of which are incorporated herein by reference.
In addition to providing focus adjustments, such systems can provide fringe-pattern-based reference signals that can be used to determine substrate shifts or apparent shifts that are unrelated to surface topography or height variations. As described in the Smith et al. references cited above, systems that provide both surface height-based focus signals and reference signals use fringe patterns that are imaged onto different areas of a common detector. Unfortunately, limiting a reference signal or height signal to a portion of a detector can limit signal sensitivity and decrease signal to noise ratio. For these reasons, improved methods and systems for distinguishing topography-based signals from reference signals are needed.